l. Field of the Invention
This invention relates to vibration generators and, more particularly, a vibration generator adapted for use in a compaction machine and capable of producing either relatively high or low amplitude of vibration over a range of frequencies.
2. Description of the Prior Art
In the vibratory compaction of materials for roads and other engineered surfaces certain qualities of the materials to be compacted make either a relatively high or low amplitude of vibration appropriate. Other factors, such as speed of the compaction machine and desired surface finish, make a higher or lower frequency of vibration appropriate. The production of a high quality surface also requires that the vibration generator be capable of starting and stopping quickly as the travel direction of the compaction machine is changed. Further, minimizing the amplitude of vibration during starting and stopping reduces the likelihood of disturbing the compacted surface. It is, therefore, desirable that the vibration generator of a compaction machine be simple and economical to manufacture, provide for easy operator selection of the appropriate amplitude and frequency, be operable at the selected amplitude throughout the range of frequencies available and be capable of rapid starting and stopping with low power requirements.
In compaction machines an unbalanced rotor is commonly utilized as a vibration generator. The rotor comprises a shaft with one or more eccentric weights attached. The frequency of vibration is varied by altering the rotational speed of the rotor. The amplitude of vibration is determined by the mass of the eccentric weight and its eccentricity, the distance from the center of mass of the eccentric weight to the rotational centerline of the rotor shaft. The amplitude of vibration is varied by altering the effective mass or the eccentricity of the eccentric weight. Likewise, the torque and, consequently, the energy to rotate the rotor varies directly with the mass and eccentricity of the eccentric weight.
A variable amplitude vibration generator utilizing a radially movable eccentric weight is known in the prior art. Brander; U.S. Pat. No. 4,033,193; discloses a variable amplitude vibration generator having a weight mounted to the rotor shaft in such a way that it can move radially from the centerline of the rotor shaft but is urged toward the rotor shaft by a spring means. When the rotor shaft is rotated, centrifugal force acts on the movable weight causing it to displaced radially increasing the eccentricity of the movable weight and increasing the amplitude of vibration. Likewise as the shaft slows the force exerted by the spring means causes the movable weight to move toward the rotor shaft reducing the vibratory amplitude and the torque necessary to rotate the rotor shaft. While this device is relatively uncomplicated and changing the amplitude of vibration is easy; vibration amplitude is frequency dependent and operation of the vibration generator at a set amplitude throughout the range of frequencies is not possible.
In the prior art various means have been utilized to control the radial positioning of the movable weight of the vibration generator. Larson; U.S. Pat. No. 3,896,677; shows a typical mechanism. However, these mechanisms have been relatively complicated and required considerable maintenance as a result of the need to make a mechanical or hydraulic connection to the movable weight through the center of the rotating and vibrating rotor shaft.